The pumps generally used to dispense liquid or semi-liquid products are so-called “needle-valve pumps”. Such pumps comprise at the top a needle valve mounted so as to move freely between a closed position in which it blocks the end opening of the pump, and prevents the dispensing of the product, and an open position in which it allows the liquid to pass through said end opening.
Such pumps, in a standard manner, comprise a first cylinder forming a pump chamber, which communicates with an intake tube by means of a check valve, and in which a hollow piston is slidingly mounted. They also comprise a second cylinder, which communicates with the first cylinder, in which a second piston formed by a needle valve is slidingly mounted, capable of interrupting the communication between the first cylinder and a dispensing nozzle. They finally comprise elastic means, such as a metallic compression spring, which act on the needle valve so as to maintain the latter in a blocking position in which said communication is interrupted.
Such pumps have the following operating principle: when the first piston is moved, the liquid located in the first cylinder is forced into the second cylinder, in which the pressure increases. The second piston tends to push into its cylinder, compressing the spring. When the pressure of the liquid reaches a high enough value to balance this action, the second piston moves, driving the valve linked to it, so that the cylinders are placed in communication with the dispensing nozzle.
Such pumps do not comply with the present demands of quality standards since, for example, the liquid product can be polluted through contact with the metal spring that returns the needle valve to blocking position after dispensing the liquid product.
Furthermore, the doses of liquid product dispensed are often inaccurate, which limits the use of these pumps to the dispensing of certain products.
In order to attempt to overcome these disadvantages, patent application WO 2006/125880, filed by the present Applicant, already describes a pump comprising a needle valve made up of a base topped by a rod, and a dispensing head manually movable on a pump body between an inactive position and a punctual product-dispensing position. The head comprises elastic means for respectively returning said needle valve to the position in which it blocks a dispensing orifice and the dispensing head to the inactive position. The return means are arranged in the dispensing head so as not to be in contact with the liquid product to be dispensed. The dispensing head also comprises a channel for allowing the liquid product to pass between the dosage chamber and a second chamber provided at the end of said dispensing head, so as to communicate with the orifice of the dispensing head.
The opening of the dispensing orifice, allowing the liquid product to be ejected towards the outside of the pump, takes place under the action of the pressure exerted by the liquid product contained in the second chamber on the needle valve. More precisely, under the action of a pressure exerted by a user on the dispensing head, the liquid product contained in the dosage chamber is compressed. The latter then exerts a pressure on the liquid product contained in the channel, and indirectly on the liquid product contained in the second chamber. The liquid product contained in the second chamber, exerting a pressure on the base of the needle valve, then compresses the elastic return means of the needle valve. Under this action, the rod of the needle valve separates from the dispensing orifice, freeing up a passage through which the liquid product is ejected to the outside of the pump.
However, this type of pump needs to be primed prior to dispensing the product, which means evacuating the air that was trapped in the dosage chamber, channel and second chamber during the assembly of the pump. The principle of air evacuation is similar to the previously described liquid-product dispensing principle. And yet, the pressure exerted by the air contained in the second dosage chamber is not always enough to separate the needle valve from the dispensing orifice and thus to open the passage for ejecting the air from the pump. This can result in difficulties when priming the pump.